1. Field of the invention
The present invention relates to roller bearing cages such as needle roller thrust bearings etc. More specifically, the invention relates to roller bearing cages such as needle roller thrust bearings etc. that are used in an automatic transmission of automobile, a compressor of a vehicle air conditioner or the like.
2. Description of the Related Art
Trend for low fuel consumption of automobile in recent years has increased demands for further weight saving and further reduction of rotational torque to the roller thrust bearings that are mounted in various parts such as automatic transmission in the vehicle more and more. For weight saving, a cage made of resin has been proposed. For reduction of rotational torque, there has been proposed a cage in which projections are formed at central parts in the circumferential direction on opposite inner faces of the cage pocket over the thickness direction of the cage (width direction of assembly) to pivotally support a roller by contacting with circumferential central portions on end faces of the roller on inner and outer circumferential sides (See Japanese Patent Publication No. JP 2004-211824A). In the cage disclosed in JP 2004-211824A, by providing the projections in opposite inner faces of the cage pocket, the reduction of rotational torque can be accomplished because contact area between the opposite inner faces and the end faces of the roller is reduced. However, in such the cage, due to limitation in machining shape of punching die for the cage pocket, the projections must extends over the thickness direction of the cage. Accordingly, a linear contact area between the projections and the end faces of the roller in the thickness direction is increased, and contact portions are exposed to a large difference in contact speeds in the thickness direction. As a result, in such the structure, abnormal wear (drilling wear) can easily occur on the inner faces of the cage pocket. As the cage disclosed in JP 2004-211824A is made of metal, the thickness of the cage can be made relatively thin, so that the length of the linear contact of the projection can be made small. However, if the cage is made of resin for saving weight, the thickness of the cage is increased in order to secure strength of the cage and also, due to limitation in shape to provide draft for releasing resin formation mold, the projection must be made to extend more in the thickness direction of the cage as compared with the metal cage. Accordingly, the difference in the contact speeds between the projection and the end face of the roller is further increased, and the drilling wear as discussed above becomes more considerable.